Place the old rocking chair cushion on a flat surface over a sheet of butcher paper.
Trace the shape of the cushion onto the paper using a marker, making the outline a 1/2 inch away from the edge of the cushion to allow room for seams.
Cut out the cushion pattern using a pair of scissors.
Use a seam ripper to tear open the old cushion and take all the stuffing out. Put the stuffing aside for later and discard the old cushion case.
Double over the fabric of your choice that you wish to reupholster the cushion with. Make sure the decorative sides are facing each other on the inside.
Secure the paper pattern to the doubled over fabric using sewing pins. Cut the shape out of the doubled fabric, and remove the sewing pins and the pattern.
Pin the two pieces of fabric together around the edges using the sewing pins.
Bind the fabric together by stitching the outside perimeter of the cushion case, about 1/2 inch in from the edge, using a needle and thread or loaded sewing machine. Stop sewing when there is 2 inches of space left open; knot the thread, and cut away the needle with the scissors.
Remove the sewing pins and turn the rocking chair cushion case right-side out through the small opening you left.
Insert the old stuffing from the cushion into the opening, adding additional cotton stuffing if necessary. Use a wooden dowel to push the stuffing into corners.
Sew the remaining 2 inches shut to complete the cushion.